PTEN Deficiency and Tumor Development PTEN is one of the most frequently mutated genes in human cancer. We have previously demonstrated that PTEN plays an essential role in the maintenance of genomic stability and that PTEN controls genome integrity through multiple mechanisms. The mitotic checkpoint is the most important mechanism for ensuring accurate chromosome segregation during cell division. Our preliminary data show that knockdown of PTEN causes severe mitotic misalignment and increases the frequency of monopolar and multipolar spindles, suggesting that PTEN deficiency impairs kinetochore congression and spindle bipolarity. Moreover, disruption of Pten eliminates the mitotic checkpoint response to spindle damage. Of significant interest was our finding that the levels of Bub1 and aurora B which are two critical mitotic checkpoint regulators are reduced in Pten null cells. In this grant application, we propose that PTEN plays a critical role in mitotic checkpoint control and spindle assembly. Our first specific aim is to characterize how PTEN deficiency affects kinetochore functions, spindle assembly and mitotic checkpoint activity. To do so, we will identify potential kinetochore or spindle factors physically associated with PTEN during mitosis. We will also determine whether wild-type PTEN can correct mitotic errors and restore the mitotic checkpoint. Our second specific aim is to determine whether the phosphatase activity of PTEN is necessary for its mitotic function and further define the functional domain of PTEN responsible for bipolar spindle assembly and the mitotic checkpoint. A variety of PTEN mutants with and without the N-terminal phosphatase domain will be tested for their ability to establish spindle bipolarity, sustain the expression levels of checkpoint proteins, and maintain a functional mitotic checkpoint. Our third specific aim is to explore the mechanism of how PTEN participates in the regulation of Bub1 and aurora B in synergy with E2F-1. Our preliminary chromatin immunoprecipitation assays identified both PTEN and E2F-1 on the mitotic gene promoter, which suggests there is functional cooperation of PTEN and E2F-1 on chromatin. We will therefore determine how PTEN acts on chromatin to modulate the transcriptional regulation of mitotic genes by E2F-1. Our final specific aim will be to further evaluate the role of PTEN in controlling the mitotic checkpoint and chromosomal stability using a Cowden syndrome model where PTEN is inherently mutated. We will examine the mitotic checkpoint activity in human lymphocytes with mutant PTEN. We will characterize the gain of function of PTEN mutants and determine whether these dominant-negative PTEN mutants disrupt the mitotic checkpoint and induce chromosome instability. Finally, we will use a PTEN189 mutant knock-in mouse model to determine whether the PTEN189 mutation causes genomic instability and results in tumorigenesis. Successful completion of this project will define PTEN as a controller of the spindle checkpoint and a guardian of the genome. New findings from this project may provide insights into the mechanism whereby PTEN deficiency and consequent mitotic dysfunction lead to tumorigenesis. PUBLIC HEALTH RELEVANCE: PTEN Deficiency and Tumor Development Narrative The PTEN tumor suppressor is frequently mutated in a variety of human cancers. Loss of PTEN leads to tumorigenesis in mouse models. The mitotic checkpoint is a major mechanism for ensuring chromosome inheritance and preventing malignancy. This project will explore novel functions of nuclear PTEN in maintaining genomic stability by revealing its critical role in spindle assembly and chromosome segregation. New findings from this study will answer the fundamental question of how PTEN deficiency impairs the mitotic surveillance machinery, leading to tumor development. Identification of the PTEN-mitotic pathway may offer a profound implication for development of therapeutic strategies against tumorigenesis.